Kei Yūki, a motorcyclist, forges an initial link to the main characters when he nearly collides with C-ko after she runs into the street during an argument with A-ko. He later encounters A-ko at her fast-food restaurant job, drawing romantic attention from both A-ko and B-ko. Despite their pursuit, he maintains a largely silent presence, his focus subtly fixed on C-ko, whom he observes from afar.

He gives A-ko rides on his motorcycle, fueling C-ko's jealousy and hostility toward him. At an alien-hosted event confrontation, he reveals his feelings for C-ko, only to face her decisive rejection. A significant detail is his possession of an earring dropped by C-ko, carefully preserved and hinting at his earlier, unnoticed attention. His favorite movie is "Tabidachi - Ami Shusho," a preference separate from the main narrative.

Later developments see him engaged to Miss Ayumi through an arranged marriage orchestrated by B-ko's father, Mr. Daitokuji. This engagement heightens the rivalry between A-ko and B-ko, both opposing the union even as an unrelated alien threat emerges. His presence consistently acts as a catalyst for interpersonal conflicts among the central characters.